Three-dimensionally reinforced cellular matrix composite and method of making same

ABSTRACT

A light-weight and impact-resistant composite material formed from a three-dimensional textile structure preform comprising at least three systems of yarns that define a plurality of interstices within the textile structure. A cellular matrix material impregnates the textile structure so as to fill the interstices of the three-dimensional textile structure and to coat at least a portion of the surface area thereof.

TECHNICAL FIELD

The present invention relates to cellular foamed materials, and moreparticularly to a three-dimensionally reinforced cellular matrixcomposite material characterized by lightweight and high impactresistance properties.

BACKGROUND

The use of high-performance composite fiber materials is becomingincreasingly common in applications such as aerospace and aircraftstructural components. As is known to those familiar with the art, fiberreinforced composites consist of a reinforcing fiber such as carbon orKEVLAR® and a surrounding matrix of epoxy resin, PEEK or the like. Mostof the well-known composite materials are formed by laminating severallayers of textile fabric, by filament winding or by cross laying oftapes of continuous filament fibers. However, all of the laminatedstructures tend to suffer from a tendency toward delamination. Thus,efforts have been made to develop three-dimensional braided, woven andknitted preforms as a solution to the delamination problems inherent inlaminated composite structures. Representative three-dimensional textilepreforms are disclosed in U.S. Pat. No. 5,465,760 issued to Mohamed etal. on Nov. 14, 1995 and U.S. Pat. No. 5,085,252 issued to Mohamed etal. on Feb. 4,1992.

Also, it is well-known to make conventional foamed materials, such asfoamed polymer plastic materials, that have microcellular voidsdistributed throughout the material. Standard techniques for thispurpose normally use chemical or physical blowing agents. For example,chemical blowing agents are low molecular weight organic compounds whichdecompose at a critical temperature and release a gas or gases such asnitrogen, carbon dioxide, or carbon monoxide. Methods using physicalagents include the introduction of a gas as a component of a polymercharge or the introduction of gases under pressure into molten polymer.These well-known and standard foaming processes produce voids or cellswithin the plastic materials which are relatively large (for example, onthe order of 100 microns or greater), as well as relatively wide rangesof void fraction percentages, for example from 20% to 90% of the parentmaterial. The number of voids per unit volume is relatively low andoften there is a generally non-uniform distribution of the cellsthroughout the foam material such that the materials tend to haverelatively low mechanical strengths and toughness. See, for example,U.S. Pat. No. 3,796,779 issued to Greenberg on Mar. 12, 1976.

It is also well-known in the foamed materials art that in order toimprove the mechanical properties of conventional cellular foammaterials, a microcellular process was developed for manufacturing foamplastics having greater cell densities and smaller cell sizes. See, forexample, U.S. Pat. No. 4,473,665 issued on Sep. 25,1984 to J. E.Martini-Vredrensky et al. The improved technique provides forpre-saturating the plastic material to be processed with a uniformconcentration of a gas under pressure and the provision of a suddeninduction of thermo-dynamic instability in order to nucleate a largenumber of cells. For example, the material can be pre-saturated with thegas and maintained under pressure at its glass transition temperature,and the material then suddenly exposed to a low pressure to nucleatecells and promote cell growth to a desired size, depending on thedesired final density, and thereby producing a foamed material havingmicrocellular voids or cells therein. The material is then quicklyfurther cooled, or quenched, to maintain the microcellular structure.

Additional work in producing microcellular foam plastic material isdescribed in U.S. Pat. No. 4,761,256 issued on Aug. 2, 1988 toHardenbrook et al. Further, U.S. Pat. No. 5,334,356 and U.S. Pat. No.5,158,986 both issued to Baldwin et al. disclose apparatus and processfor forming a supermicrocellular foamed material having cellsdistributed throughout the material with average cell sizes being atleast less than 2.0 microns and preferably in a range from about 0.1micron to about 1.0 micron.

Although all of the above is well-known to those skilled in the textilearts and microcellular foamed material arts, applicants have recognizedthe need for an improved three-dimensional composite material which doesnot tend to delaminate and that possesses lightweight and very highimpact resistance. Toward that end, applicants have developed a newthree-dimensionally reinforced cellular matrix composite and the methodfor making the product that allows for the formation ofthree-dimensional reinforced composites with a cellular matrix thatcontains intentionally induced voids. The voids render the compositematerial extremely light in weight while simultaneously providingenhanced impact resistance and enhanced specific bending stiffness.Applicants believe that this novel composite material is new in thecomposite art and meets a long-felt need for such a product and a methodfor making the product.

Summarily, applicants have discovered a novel three-dimensionallyreinforced cellular matrix composite and a method for making the samethat combines cellular technology with three-dimensional textile preformtechnology in order to provide a novel lightweight composite materialwith superior structural integrity.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, applicants provide alightweight and impact resistant composite material comprising athree-dimensional textile structure preform formed of at least threesystems of yarns that define a plurality of interstices within thetextile structure. A cellular matrix material fills the interstices ofthe three-dimensional textile structures and coats at least a portion ofthe surface area of the three-dimensional textile structure.

In accordance with another aspect of the present invention, applicantsprovide a method of producing a three-dimensionally reinforced cellularmatrix composite including providing a three-dimensional textilestructure preform formed of at least three systems of yarns that definea plurality of interstices within the textile structure. Next, afoamable polymer material is introduced to the three-dimensional textilestructure preform so as to fill the interstices and impregnate thethree-dimensional textile structure preform and to coat at least aportion of the surface area of the structure. The foamable polymermaterial is then foamed to produce a microcellular foamed polymermaterial containing a plurality of voids or cells distributedsubstantially throughout the foamable polymer material.

It is therefore an object of the present invention to provide athree-dimensionally reinforced cellular matrix composite that islightweight and impact resistant.

It is another object of the present invention to provide athree-dimensionally reinforced cellular matrix composite thatincorporates a three-dimensional textile structure preform in order toprovide enhanced structural integrity.

It is another object of the present invention to provide athree-dimensionally reinforced cellular matrix composite thatincorporates a three-dimensional textile structure preform to provideenhanced performance characteristics including enhanced resistance todelamination.

It is still another object of the present invention to provide athree-dimensionally reinforced cellular matrix composite thatincorporates a three-dimensional textile structure preform and thatprovides enhanced resistance to delamination, enhanced impactresistance, enhanced fatigue life, enhanced strength-to-weight ratio,and enhanced stiffness-to-weight ratio.

Some of the objects of the invention having been stated, other objectswill become apparent with reference to the drawings describedhereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of the three-dimensionally reinforced cellularmatrix composite of the invention;

FIGS. 2A-2C show a perspective, side and top view, respectively, of athree-dimensional orthogonally woven preform which is utilized in oneembodiment of the three-dimensionally reinforced cellular matrixcomposite of the present invention;

FIG. 3 is a photomicrograph view in the Z yarn direction of thethree-dimensionally reinforced cellular matrix composite of the presentinvention;

FIG. 4 is a photomicrograph view in the Y yarn direction of thethree-dimensionally reinforced cellular matrix composite of the presentinvention;

FIG. 5 is a photomicrograph view in the X yarn direction of thethree-dimensionally reinforced cellular matrix composite of the presentinvention;

FIG. 6 is a schematic view of a testing apparatus for making thethree-dimensionally reinforced cellular matrix composite of the presentinvention;

FIG. 7 is a chart illustrating tensile test results of thethree-dimensionally reinforced cellular matrix composite of the presentinvention;

FIG. 8 is a chart illustrating tensile test results of a controlcomposite impregnated with pure (no voids) epoxy resin;

FIG. 9 is a chart illustrating load versus deflection curves in threepoint bending test of the three-dimensionally reinforced cellular matrixcomposite of the present invention; and

FIG. 10 is a chart illustrating load versus deflection curves in threepoint bending test of a control composite material impregnated with pure(no voids) epoxy resin.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIGS. 1-10 of the drawings, applicants wish to describethe three-dimensionally reinforced cellular matrix composite C andmethod of making the same of the present invention. Although FIG. 2illustrates an orthogonally woven three-dimensional preform, theinvention is not intended to be limited to this structure but to includeother three-dimensional textile structure preforms formed of at leastthree systems of yarns so as to provide interstices within the structureand structural integrity to composite C. These structures can includewoven, braided, circular woven and knitted three-dimensional structuresformed of at least three different yarn systems.

Applicants have developed a novel three-dimensionally reinforcedcomposite C and novel method for making the composite such that thethree-dimensionally reinforced composite contains voids of various sizesin the matrix. The uniqueness of applicants' inventive product andprocess is the combination of known cellular technology with knownthree-dimensional textile preforms with inherent structural integrity.The unexpected and surprising result is a novel composite that hassignificantly less specific density (e.g., 0.88 to 1.02 grams/cm³) thana conventional three-dimensional composite (e.g., 1.44 grams/cm³) andthat has higher impact-resistance, higher fatigue resistance, higherspecific tensile strength, and compression strength, higher specifictensile modulus, and higher specific bending stiffness.

A. Theoretic Basis of the Composite

Formation of voids in epoxy matrix may be fulfilled with eitherchemicals or simple gases as blowing agents. In order to avoidinteraction of chemical blowing agent on the fiber/matrix system andalso to control the size of the voids, simple gases are used inapplicants' testing of the invention. The foaming process of a thermosetmatrix consists of three stages: gas saturation, nucleation of cells andcell growth. The matrix is first saturated with simple gases such as CO₂and N₂ under high pressure (1000-3000 psi). At a certain viscosityduring the matrix curing process, the pressure is abruptly released,creating a thermodynamic instability, which leads to nucleation andgrowth of cells or bubbles. The matrix viscosity, when the pressure isreleased, determines the bubble size. The cells typically define a voiddiameter of between about 0.01 to 10.0 μm, but cell void diameter can belarger.

Nucleation of gaseous cells in the matrix of fiber-reinforced compositeC can occur under three mechanisms: (1) homogenous nucleation occurringin the homogeneous polymer phase, (2) heterogeneous nucleation occurringat the interface of polymer and filler (e.g., particles, fibers, etc.)phases, and (3) a combined mode of (1) and (2). A reversible process ofthe homogeneous nucleation of a gaseous cell can be described by thefollowing equation:

ΔG _(hom) =−V _(b) ΔP+A _(bm)γ_(bm)

where ΔG_(hom) is the Gibbs free energy for homogeneous nucleation,V_(b) is the volume of the bubble nucleus, ΔP is the pressure changewhich can be as high as a few thousand psi when simple gases are theblowing agents, A_(bm) is the surface area of the bubble nucleus, andY_(bm) is the surface energy of the bubble-polymer interface.

For a heterogeneous nucleation process where bubbles nucleate at theinterface of reinforcing fibers and the matrix, the Gibbs free energyneeded to nucleate a critical nucleus can be found with the followingequation:

ΔG* _(het) =ΔG* _(hmo) S(Θ)

where${S(\Theta)} = {\frac{1}{4}\left( {2 + {\cos \quad \Theta}} \right)\quad \left( {1 - {\cos \quad \Theta}} \right)^{2}}$

and θ is the wetting angle. Therefore, the rate of heterogenousnucleation is as follows:$N_{het} = {N_{1}{\exp \left( \frac{{- \Delta}\quad G_{het}^{*}}{kT} \right)}}$

For a combined nucleation mode, applicants joined nucleation rate iscomputed as follows:

N=N _(het) +ωN _(hom)

where ω is a reduction factor to N_(hom) due to N_(het).

The growth of the bubbles by gas diffusion can be presented by a massbalance at the bubble surface as follows:${\frac{}{t}\left( {\frac{4}{3}\quad \pi \quad r_{b}^{3}C_{b}} \right)} = {{4\quad \pi \quad r_{b}^{2}{D\left( \frac{\partial C}{\partial r} \right)}r} = r_{b}}$

which is subjected to the following boundary conditions:C(t, r_(b)) = P/m$\frac{\partial{C\left( {t,r_{m}} \right)}}{\partial r} = 0$

where C is the concentration of the gas in the polymer matrix, C_(b) isthat gas concentration in the bubble, D is the diffusion coefficient ofthe gas in the matrix, m is the Henry's constant, r_(b) is the bubbleradius and r_(m) is the radius of the diffusion boundary.

In the case of a thermosetting resin being used, the process of bubbleformation, including nucleation and growth, is in conjunction with thecure of the resin. The reaction rate equation may be written as follows:${- r_{R}} = {\frac{C_{R}}{t} = {k_{R}C_{R}^{\alpha}}}$

where r_(R) is the rate of the consumption of the resin, α is the orderof the reaction with respect to the concentration of the resin, C_(R),and k_(R) is the rate constant.

Since the resin becomes more and more viscous during polymerization andfinally solidifies, the diffusion of the gas is then gradually reducedand therefore the growth of the bubble gets slower or even ceases. Thus,the process needs to be controlled such that the bubble formation beginsafter the viscosity of the resin has reached a certain value so that thebubbles would not coalesce, and completes before the resin has fullysolidified.

The diffusion coefficient of the gas in the matrix is a function of thegas concentration is:

D=D ₀exp(aC)

where a is the plasticizing factor, D₀ is the zero concentrationdiffusivity. D is temperature dependent and is also, in the case ofthermosetting resin being used, affected by the changing viscosity ofthe resin during cure.

Fabricating and Testing Procedures for Development of Composite C

The 3-D fabric can be fabricated on one of the 3-D weaving machineslocated in the 3-D Weaving Laboratory of the College of Textiles ofNorth Carolina State University in Raleigh, N.C. The fabric preform willbe impregnated in epoxy resin and cured to form composite samples.

In order to fabricate 3-D woven fabric composites C of desiredproperties with appropriate bubble sizes and volume fraction, it isnecessary to investigate the effects of properties of the materialsemployed for making composites and operation conditions, such assolubility and diffusivity of the gas in the matrix material, theoperating temperature and pressure, and the batch operating time on thebubble formation process, including bubble nucleation, bubble growth bydiffusional mass transfer, as well as their effects on the cure of theresin which in turn has an influence on the diffusion of the gas. Withthe following experiments, applicants were able to acquire the datanecessary for understanding and controlling the fabrication process toproduce the required composites C.

1. Gas Solubility and Diffusivity in Matrix Material.

To determine gas diffusivity within a thermosetting resin, a bubble of agiven gas is injected into the resin and then observed under an opticalmicroscope. Due to the diffusion of the gas from the bubble into theresin, the radius of the bubble gradually decreases. By this experiment,diffusivities of the gas in the resin of variant degree ofpolymerization can be obtained. The solubility can be acquired with agas-saturated sample by means of gas-chromatography (GC) or highpressure liquid-chromatography (HPLC).

2. Cure Kinetics of Thermosetting Resin.

Infrared spectroscopic investigation is to be employed to determine theresin cure kinetics so that the duration time during which resin isgetting solidified with significant gas diffusion can be obtained. Insitu IR spectroscopy enables concentration of individual chemicalspecies to be obtained in real time. Resins are also quenched at varioustimes to terminate the reactions for the determination of cureconversion, and the measurement of the gas diffusivity and the viscosityof the intermediate product. The viscosity and conversion parameters areto be correlated with the diffusivity.

3. Bubble Formation.

In the case of thermosetting polymer being used to form the matrix, the3-D fabric preform is first impregnated with the resin and then bubblesare to be formed before the complete cure of the resin. In addition tothe pressure control and gas saturation and release temperature control,monitoring of the cure time of the resin serves as another important keyto bubble formation. The nucleated bubbles will grow up to almostcomplete cure of the resin. The amount of the bubble formed aredetermined by the amount of gas released during the time that thecomprising processes concur, i.e., the nucleation of the bubbles, thediffusional mass transfer of the gas across the interface, and the curereaction of the resin.

Bubble size and total volume fraction in the matrix can be observed witha microscope and measured by means of stereological micrograph analysis.The difference in gas solubility in the polymer/resin under thedifferent conditions before and after bubble formation represents thedriving force for the process, and the solubility data can be used topredict the total amount of the gas to be released to form bubbles.While the actual number of the bubbles nucleated depends on theactivation energy involved in overcoming the barrier for nucleation, andthe sizes of the bubbles depend on how much gas will transfer bydiffusion from the matrix to each of these bubble sites.

4. Process Control and Modeling.

The pattern of the bubbles formed in the matrix depends on thetrajectories of the temperature and pressure during the whole process ofbubble formation. Experimental data are processed with the theoreticalequations to set forth herein to acquire the relationships between theoperation parameters (such as the temperatures, the pressures, and batchoperation time) and the matrix features of the composites (bubblefraction, bubble sizes and size distribution) for the process controland modeling. With this knowledge, applicants were able to optimize theformation of experimental composites with the desired properties. Sincethese processes are highly non-linear, a novel optimization method,genetic algorithm, will be used to search the optimal solution to thisproblem.

5. Mechanical Property Testing.

Tensile and compression tests can be performed in both composite warpand filling (planar) directions. The tensile test can be carried outaccording to ASTM Standard D3039-76 [8] on an INSTRON machine. Thecompression test can be conducted following ASTM Standard D3410-87. Ingeneral, shear properties of composites C are tested using standard testmethods such as the conventional short beam bending test. For 3-Dcomposites, since no delamination occurs, applicants developed a uniquetest to test their interlaminar shear strength using a special shearfixture. This method was used in the experimental testing of the novelcomposite C.

B. Experimental Testing Apparatus and Procedures

One unique characteristic of the fabrication of the novel composite C isthe foaming of the epoxy resin matrix. Foaming by free expansion ofepoxy resin was carried out by applicants using the experimentalapparatus shown in FIG. 6, including the following testing equipment:

FIG. 6 Apparatus for Foaming of Epoxy Resin.

1. High-Stirred High-Temperature/High-Pressure Vessel available fromParr Instrument Company (Moline, Ill.) Model 4672. Maximum pressure:3,000 psi (207 bar); maximum temperature: 600° C.; diameter: 5.5 in.;depth: 9.95 in.; volume: 1 gallon.

2. Floor Stand Ceramic Heater available from Parr Instrument Company(Moline, Ill.) Model 4933 Wattage 3,000; volume: 1 gallon.

3. Digital Temperature Controller available from Parr Instrument Company(Moline, Ill.) Model 4842.

4. High Inlet/Outlet Pressure Regulator available from Matheson GasProducts (Morrow, Ga.) Model 3064-677. Delivery pressure: 200-6,000 psig(13.6-408.2 bar); delivery pressure gauge: 0-5,000 psig (0-340.1 bar);cylinder pressure gauge: 0-7,500 psig (0-510.2 bar).

5. Pressure Hose Assembly available from Parr Instrument Company(Moline, Ill.) Model A495HC. Maximum pressure: 2,500 psig (170 bar);length 6 ft.

Testing Procedures

1. Preparation of epoxy solution: EPON Resin 9405 and EPI-CURE 9470Curing Agent were weighed and added into a container with the mix ratiobeing 100/28, then stirred to form a homogeneous solution.

2. Preparation of fabric preform: a 3-D woven carbon fabric preform wascut to the desired size and weighed. The fabric size was 4.0×8.0 inches.Due to the limited availability of fabric, two types of fabric withdifferent fabric parameters were used for making composites. Fabricparameters are listed in Table 1 below.

TABLE 1 Parameters of 3-D Woven Carbon Fabric Preforms Warp (x) Weft (y)Thickness Label yarn yarn Z-yarn Ends/in Picks/in (mm) TM 12 K 6 K 3 K15 24 4.57 TS 12 K 6 K 3 K 14 20 4.50

3. Wetting of fabric preform with resin: the fabric preform was soakedin the epoxy resin solution and vacuumed (PRECISION SCIENTIFIC vacuumpump Model DD-100) at 100° C. (ISOTEMP® vacuum oven, Fisher ScientificModel 285A) for several minutes (e.g., 5-10 minutes) to expel airbubbles and ensure fully wetting of fibers with the resin.

4. Molding: the resin-impregnated preform was removed from the resinsolution and wrapped up with bleeder and release fabric, for removal ofexcess resin from the surface of the preform and for ease of removal ofthe composite C from the mold, respectively. The Breather & Bleederfabric used was AIRWEAVE® S, and the release fabric (or peel ply) wasRELEASE EASE 234 TFP, both from AIRTECH International, Inc. This packagewas placed in between the two plates of the mold. The plates have aplurality of holes to allow gaseous blowing agents to penetrate into thewoven fabric preform through the plates as well as the bleeder andrelease fabrics. The mold was then assembled by tightening the screws.

5. Loading of vessel with mold: the mold was placed in the vessel andthen the vessel head was sealed by tightening the cap screws using atorque wrench (Sears CRAFTSMAN® MICROTORK® torque wrench Model 44541) ata torque of 35 ft-lbs. This 35 ft-lbs. torque was required for the Model4672 high-pressure vessel of 3,000 psi, although the working pressurewas usually lower than this value. Then, the vent valve was closed andthe inlet valve was opened. To prolong the lifetime of the graphitegasket (Parr Instrument Company Model 1812 HCKL) and ensure good sealingof the vessel, the gasket was lubricated with KEL 110 silicone spray(available from Kellogg's Professional Products, Inc.) before assemblingthe vessel head.

6. Filling gaseous blowing agent: the deliver valve was opened to letthe gaseous blowing agent (e.g., nitrogen gas) fill in the vessel untilthe pressure reaches a required value, in a range from 20 to 120 bars,then close the inlet of the vessel. Work was done as the gas flowed intothe vessel and consequently the temperature of the system increased by afew degrees (e.g., 5-15° C., depending on the value of pressure).

7. Absorption of gas into resin: the absorption of the gas into theresin solution began once the gas had been introduced into the vessel.Then, the heater was turned on and the vessel temperature increased fromroom temperature to a moderate temperature T₁. Timing was started andthe temperature kept constant for t₁ hours as required. Fluctuation ofthe temperature was 1° C. about T₁. To enhance the diffusion of the gasmolecules into the resin and to ensure saturation of the resin with thegas, T₁=40° C. and t₁=12 hours were selected. During this time, curingof the resin proceeded very slowly.

8. Curing and foaming of resin: once time reached t₁, the heater was setat a higher temperature T₂ (e.g., 100° C.) which allowed the resinsystem to cure much faster. The system was maintained at T₂ fort₂=1.5-2.5 hours (before complete cure). Bubble nucleation was triggeredby a sudden pressure quench when the time of curing reached t₂. Due tothe small diameter of the outlet from the vessel head, this degassingprocess usually lasts for about 30-60 seconds. Excess resin that residedin empty pockets of the 3-D woven carbon were blown out of the preformand absorbed by the bleeder fabric. Before the heater was turned off,the system temperature was maintained at T₂ for several hours (e.g., 4more hours) to allow complete cure of the foamed resin.

9. Removal of the composite: after post cure for about 3-4 hours, thevessel was opened and the mold was removed from the vessel. Thecomposite sample was removed from the mold, and then weighed andlabeled. Testing specimens for tensile, bending and impact tests wereprepared from the samples.

Fabrication of 3-D Woven Carbon Fabric Reinforced Composites withRegular (Non-Foamed) Epoxy Matrix

The purpose of fabrication of 3-D woven carbon fabric reinforcedcomposites with regular epoxy matrix was to provide comparison for theinventive composites C fabricated by the procedures described above.Procedures of fabrication of “regular” composites were the same as thefirst several steps for the new type of composites, but without theprocess steps of absorption of gas and foaming of epoxy:

1. Preparation of epoxy solution: EPON Resin 9405 and EPI-CURE 9470Curing Agent were weighed and added into a container with the mix ratiobeing 100/28, and then stirred to form a homogeneous solution.

2. Preparation of fabric preform: a 3-D woven carbon fabric preform wascut to the desired size and weighed. The fabric parameters anddimensions of the preform were the same as those for the new compositesC, as listed in Table 1.

3. Wetting of fabric preform with resin: the fabric preform was soakedin the epoxy resin solution and vacuumed (Precision Scientific vacuumpump Model DD-100) at 100° C. in an oven (ISOTEMP® vacuum oven, FisherScientific Model 285A) for a few minutes (e.g., 5-10 minutes) to expelair bubbles and ensure fully wetting of fibers with resin.

4. Molding: the resin-impregnated preform was wrapped with releasefabric (for ease of un-molding from the mold) and placed in a mold thatconsisted of two aluminum plates and spacer bars controlling thethickness of the composite sample. The release fabric (or peel ply) wasRELEASE EASE 234 TFP available from AIRTECH International, Inc.

5. Curing of epoxy resin: the mold system was placed in the oven at 100°C. for at least 4 hours to allow complete cure of the epoxy resin.

6. Removal: composite samples were removed from the mold, and thenweighed and labeled. Testing specimens for tensile, bending and impacttests were prepared from samples.

C. Test Results

Tensile Test (FIGS. 7 and 8)

Referring to FIGS. 7 and 8, tensile test results showed that the meantensile strength of the cellular matrix composite C is about 10.5 GPawhile that of the control group is about 9.8 GPa with the same amount offibers but 20% more cross-sectional area covered by pure epoxy resin.Using the simple Rule of Mixture (ROM), the difference in tensilestrength between the two groups was less than 10%. Since the 3-D CMC is30% or more lighter than the control composite, its strength per unitmass is about 29% higher than that of the control composite. The moduliof the two composites are approximately the same, namely around 50 GPaon average. Therefore, the modulus per unit weight is about 40% higherfor the 3DCMC composite.

Three-Point Bending Test (FIGS. 9 and 10)

Referring to FIGS. 9 and 10, the load versus deflection curves of the3-point bending test of the two composites shows that the mode offailure changed for the 3-D CMC composite from brittle failure of thecontrol group to failure of individual layers. The control compositefailed with a single peak, indicating that once a crack formed, ittraveled through the whole cross-section resulting in a catastrophicfailure. For the 3-D CMC, a pseudo ductile failure curve was evidencedwith one peak, representing the failure of the outermost layer, followedby a much wider second peak at a much greater deflection. It was obviousthat the crack developed in the first layer did not directly travelthrough the matrix to reach the second layer. The cells in the structurestopped the propagation of the crack. The energy, which is proportionalto the area under the load-deflection curve, absorbed by the 3-D CMC wasmuch higher than that absorbed by the control composite due to thisfailure mode change.

It will be understood that various details of the invention may bechanged without departing from the scope of the invention. Furthermore,the foregoing description is for the purpose of illustration only, andnot for the purpose of limitation—the invention being defined by theclaims.

What is claimed is:
 1. A lightweight and impact resistant compositematerial comprising: (a) a substantially non-compressible uni-layerthree-dimensional textile structure preform formed of at least threemutually perpendicular systems of yarns that define a plurality ofinterstices within said textile structure; and (b) a cellular matrixmaterial that substantially fills the interstices of saidthree-dimensional textile structure and serves to create voids therein.2. The lightweight and impact resistant composite material according toclaim 1, wherein said three-dimensional textile structure preform is athree-dimensional fabric.
 3. The lightweight and impact resistantcomposite material according to claim 2, wherein the three-dimensionalfabric comprises three orthogonally woven yarn systems.
 4. Thelightweight and impact resistant composite material according to claim2, wherein the three-dimensional fabric comprises a plurality of braidedyarn systems.
 5. The lightweight and impact resistant composite materialaccording to claim 2, wherein the three-dimensional fabric comprises aplurality of circular woven yarn systems.
 6. The lightweight and impactresistant composite material according to claim 2, wherein thethree-dimensional fabric comprises a plurality of biaxial weft knit yarnsystems.
 7. The lightweight and impact resistant composite materialaccording to claim 1, wherein said cellular matrix material comprises apolymer.
 8. The lightweight and impact resistant composite materialaccording to claim 7, wherein said polymer comprises an epoxy resin. 9.The lightweight and impact resistant composite material according toclaim 8, wherein said epoxy resin comprises a thermosetting epoxy resin.10. The lightweight and impact resistant composite material according toclaim 1, wherein the cellular matrix material comprises bubbles defininga void diameter of at least about 0.01 to 10.0 μm.
 11. The lightweightand impact resistant composite material according to claim 1, whereinthe composite material has a specific density of about 0.88 to 1.02grams/cm³.